Preheat oven to 275˚F. Line 2 baking sheets with aluminum foil and spray with coconut oil. Scrub beets and sweet potatoes with a vegetable brush. Using a mandolin, thinly slice the beets and sweet potatoes. Lay slices flat on the baking sheets. Cook them for about 8 minutes, then flip them and cook for another 8 minutes (or until they are crisp).